1. Field of the Invention
The invention pertains to a vibration damper of the type having a piston rod guided with freedom of axial movement in a cylinder; a first piston fixed to the piston rod so that the first piston is stationary with respect to the piston rod, the first piston bounding a first working space facing away from the piston rod; and a second piston mounted on the piston rod with freedom to move axially with respect to the first piston against the force of at least one restoring spring, the second piston bounding a second working space facing the piston rod and an intermediate working space between the pistons; and at least two openings in each piston connecting the working spaces, the openings being equipped with non-return valves.
2. Description of the Related Art
A vibration damper with a cylinder in which a piston rod is guided with freedom of axial movement, where a first piston is mounted in a stationary manner on the piston rod and a second piston is mounted with freedom to move axially on the piston rod against an elastic force, is known from the U.S. Pat. No. 6,651,787, which represents the state of the art. The two pistons divide the cylinder into a working space on the side of the second piston facing the piston rod, a working space on the side of the first piston facing away from the piston rod, and a working space between the two pistons. Valve-equipped through-openings control a connection between the working spaces. The second piston has at least one restoring spring on each side and is supported so that it can move in either of the two axial directions against the force of the restoring springs.
This vibration damper design offers very comfortable driving behavior in many types of vehicles. There are applications, however, in which the forces generated by the restoring springs are too weak, as a result of which each of the restoring springs is formed by two springs arranged one inside the other to form a set. These sets of springs, however, are difficult to install and occupy a relatively large amount of radial space. Springs with a progressive spring characteristic are very expensive.